1. Field of the Invention
This invention relates to a read out method for a gas discharge display panel, and specifically a new method and apparatus for detecting the coordinate location of a light-detecting pen on an AC plasma display panel.
2. Description of the Prior Art
Typically, a gas discharge display panel having plural row (X) and column (Y) electrodes coated with dielectric layers, arranged face-to-face across a gap filled with a discharge gas is well known as an AC plasma display panel. When such AC plasma display panels are employed in computer terminals, it is often desirable to achieve man-machine communication by using a light-pen. This type of communication is well known in the case of existing terminals utilizing cathode ray tube displays. However, AC plasma display panels have inherent memory functions so that display information, once memorized is displayed repeatedly by means of applying sustain voltage pulses. Therefore, with respect to plasma display panels, the recognition of the coordinate location of a light-pen by making use of a time scanning sequence, as is done by an electron beam in CRT displays, has not been achieved.
In the past, various concepts have been proposed for employing a light-pen detection system in conjunction with a plasma display panel. One such proposal for using a light-pen is disclosed in U.S. Pat. No. 3,651,509 issued to D. T. Ngo. In the Ngo patent, it is proposed that each cell is scanned sequentially by an inquiry signal made up of a paired write scanning pulse followed by an erase scanning pulse.
Another proposal is described in a paper titled "Light-Pen Detection with a Plasma Display Panel" reported by T. N. Criscimagna et al., IEEE Transactions, Vol. ED-22, No. 9, September, 1975, page 796 to 799. According to the Criscimagna et al. proposal, in order to prevent the influence of a reduced sustain margin due to the use of a particularly prepared inquiry signal, a pulse equivalent to the sustain voltage pulse is used as the ideal inquiry signal.
However, the existing prior art techniques have the following disadvantages: the system configuration is quite complicated and the control of driving timing of each circuit is also complicated. On the other hand, although the reading of an "on" cell can be done through the application of a selective sustain voltage pulse as will be discussed below, it has been impossible to read out the information of an "off" cell by such a sustain pulse.